In recent years there has been a great deal of interest and progress in the integration of implantable medical devices such as defibrillators and pacemakers. For the purpose of this application, "defibrillation" is used in a broad sense, as including the application of relatively high energy and high voltage shocks to the heart to terminate tachyarrhythmias including fibrillation and malignant tachycardias. Similarly, "pacing" is used in a broad sense, as including the application of relatively low energy and low voltage pacing pulses to maintain an adequate heart rate or to break a tachycardia by stimulating the patient's heart.
It is well known that automatic capture algorithms ensure that capture is maintained even in the presence of changing physiologic conditions and drugs. In view of the direction of current implantable cardioverter/defibrillator (ICD) designs, it would be desirable to offer automatic capture pacing in a combination device having pacing and defibrillating functionality. It is also well known that automatic capture requires "true" bipolar sensing, that is, tip to ring sensing, or at least unipolar sensing from the bipolar ring electrode to the case electrode.
One traditional approach to combining pacing and true bipolar sensing electrodes in a defibrillation lead is to provide a ring electrode located between the pacing tip electrode and the defibrillation electrode where the ring electrode is dedicated exclusively to sensing the heart's electrical activities. However, the optimal spacing for autocapture sensing has been found to be in the range of 1.7-3.0 (and preferable about 2.5 cm) from the tip electrode to the ring electrode. In an ICD lead, it is desirable to have the shocking coil as close as possible to the RV apex to lower defibrillation thresholds. ICD leads strive to have the shocking coil spaced down about 1.2 cm from the end. The space required for this ring electrode forces the defibrillation electrode to be set back from the RV apex, and, because of the size limitations of the right ventricle, decreases the length available for the defibrillation electrode. This would severely increase fibrillation thresholds.
However, in the context of endocardial ventricular leads, it would be desirable to provide an electrode, or electrode pair, for sensing adjacent the ventricular apex, while still providing an electrode which also is located as close to the apex as possible. Exemplary attempts to accomplish such an objective are described in U.S. Pat. No. 5,336,253, to Gordon et al. and U.S. Pat. No. 5,342,414 to Mehra.
The Gordon et al. patent describes a combined pacing and cardioversion lead system with internal electrical switching components for unipolar or bipolar sensing of electrograms, pacing at normal pacing voltages and cardioversion or defibrillation. In the bipolar embodiments, a ring electrode is coupled through the switching circuitry to a large surface area cardioversion electrode. In these embodiments, pacing and sensing are accomplished though a pair of conductors extending through the lead body to the tip and ring electrodes. When cardioversion shocks are delivered to the ring electrode, cardioversion energy is also directed to the cardioversion electrode through the operation of the switching circuitry in response to the magnitude of the applied cardioversion pulse. However, for optimal automatic capture sensing the ring electrode should be spaced approximately 2.5 cm from the tip, as noted above. This would place the composite defibrillation electrode, as taught by Gordon, 2.5 cm from the tip, which would increase defibrillation thresholds.
The transvenous defibrillation lead described in the Mehra patent is directed towards optimizing the size, spacing and location of the electrodes, and more specifically towards providing a bipolar sensing pair of electrodes having adequate interelectrode spacing to insure appropriate sensing of cardiac depolarization, while still allowing the placement of the electrode as close to the distal end of the lead body as possible. The lead includes a helical electrode, extending distally from the lead body, for use as the active electrode in cardiac pacing and for use in sensing cardiac depolarizations. A ring electrode is located at or adjacent to the distal end of the lead body and provides the second electrode for use in sensing depolarizations. The helical electrode is insulated from the point it exits the lead body until a point adjacent to is distal end. The defibrillation electrode is mounted with its distal end closely adjacent to the distal end of the lead body, such that its distal end point is within one centimeter of the distal end of the lead body. While the Mehra reference seems to address the needs of placing the defibrillation coil closer to the apex, it neglects the need to place the pacing ring electrode about 2.5 cm away from the pacing tip electrode.
U.S. Pat. No. 4,355,646 to Kallok et al. teaches the use of two rigid equal width electrodes placed in the right ventricle. However, the efficacy of this construction is questioned because it is optimized for sensing contractions via impedance changes. It could not reliably electrically sense post-shock since it requires full force shocking through the most distally located electrode. To the knowledge of the applicant, this concept has never been practiced commercially.
The leads described in the foregoing Gordon et al., Mehra, and Kallok et al. patents do not provide for "integrated electrogram bipolar sensing", wherein sensing is carried out between the defibrillation shocking coil electrode and the tip electrode. One feature that distinguishes integrated bipolar sensing and true bipolar sensing (i.e., pacing ring-to-tip sensing) is that integrated bipolar sensing lacks a ring electrode dedicated solely to bipolar sensing in conjunction with the pacing tip.
There are two potential problems with integrated bipolar electrodes. First, because the reference electrode must be large for efficient delivery of defibrillation or cardioversion energy, it may reduce the resolution of the sensed signal due to spatial averaging of the different potentials within the heart. Secondly, this electrode serves also as a defibrillation electrode and is likely to have substantial residual charge at its interface after a defibrillation therapy pulse. The residual charge or polarization of the electrodes results in less accurate sensing immediately after therapy. The true bipolar sense lead should not be subject to these potential problems. The size of the true bipolar reference electrode is not governed by the need for efficient energy delivery during therapy and can be optimized for sensing.
Additionally, because a negligible current flows across the electrode tissue interface, there is no build-up of charge or polarization at the interface, enabling the accurate measurement of endocardial signals immediately following therapy. However, a drawback with true bipolar sensing exists because the reference sense electrode in a true bipolar lead is located adjacent to the pacing electrode, and thus the defibrillation shocking coil electrode is generally positioned farther away from the apex of the heart, thus disadvantageously reducing the delivered therapeutic energy.
More recently, the invention disclosed in U.S. Pat. No. 5,545,183 to Altman is directed towards providing a method and apparatus for using a defibrillation lead to defibrillate and sense in close proximity to the heart ventricular apex. In particular, the invention is directed towards providing a new method which permits the optimal delivery of defibrillation and cardioversion energies, and the minimization of poor sensing due to polarization effect, by the selective use of the ring in parallel with the defibrillation coil to assist with difficult defibrillations. This has the same disadvantages as does the Gordon reference. That is, in order for the bipolar ring electrode to function properly for automatic capture, it must be placed about 2.5 cm away from the tip, which would put the defibrillation coil electrode even further away under normal operating conditions, and switchably to 2.5 cm only when the first defibrillation shock has failed.
It was with knowledge of the foregoing state of the technology that the present invention has been conceived and is now reduced to practice.